When ball and socket joints such as hip joints and shoulder joints are damaged, it is common to replace the entire joint with a joint prosthesis. In the case of a damaged hip joint, replacement involves resection of the proximal femur and implantation of the femoral component of an orthopedic joint, which includes a stem part which can be received in the intramedullary canal, and a head part with a convex bearing surface. The patient's acetabulum is prepared to receive the acetabular component of the joint prosthesis, which provides a concave bearing surface to articulate with the bearing surface on the femoral component. Frequently, bone cement is used to affix the components of the prosthesis within their respective prepared bone cavities.
When the condition of the femoral bone tissue is generally good, it can be desirable to retain much of the proximal femur. Accordingly, techniques have been developed in which the femoral head is fitted within a hollow resurfacing shell. The resurfacing shell has a convex outer surface which is highly polished which enables it to act against the hollow bearing surface of an acetabular component. Such techniques are referred to as Articular Surface Replacement techniques. They have the advantage that the quantity of bone which has to be removed from the head of the bone is only small. A tool which can be used to prepare the head in this way is disclosed in WO-2004/032767.
A bone cement can be used to ensure that an appropriate bond is formed between a resurfacing shell and the prepared head of a femur or other bone. Certain known techniques involve the use of cements having relatively low viscosities, and the techniques involve providing a quantity of the cement within the resurfacing shell, which is then fitted on to the prepared head. Reception of the head within the shell causes the cement to be displaced. Alternatively, the bone cement is applied to the head of the bone prior to the fixation of the implant. The use of low viscosity bone cements enables good penetration of the cancellous bone and ensures that the cement can be displaced when the shell was fitted on to the head of the bone, so that the shell can be properly seated.
The use of low viscosity bone cement for fixing a re-surfacing implant has a number of disadvantages. These include that the low viscosity cement has to be mixed and applied to the prepared head of a bone in a very short time period, as it has been found that such cements typically take only 20 minutes to set after mixing. The surgical team is therefore placed under pressure and the flexibility of the technique is limited. Low viscosity bone cement has the further disadvantage that once the cement has been applied to the head of the bone the implant must be carefully held in place until the bone cement has set. Furthermore, it has been found that the pressurisation of low viscosity cement is difficult to sustain and control. This lack of control can sometimes lead to inadequate fixation of the bone cement to the head of the bone.
The use of low viscosity cements in the fixation of articular surface prosthesis components has been found to promote a high degree of penetration of the cement into the bone beneath the pole of the implant, but with a progressive reduction in the degree of penetration of the cement towards the periphery of the implant. This variation in the degree of the penetration of the cement into the bone at the head of the bone can lead to a number of problems. Firstly, if there is a high mass of bone cement in the region of the pole of the implant this may lead to an increased temperature in the bone cement during the setting period, which may give rise to elevated bone necrosis in the region of the pole of the implant. Additionally, an excess of bone cement in the polar region of the head of the bone can in some circumstances prevent the re-surfacing prosthesis from being properly seated on the head of the bone when the re-surfacing prosthesis is fitted. Furthermore a lower degree of penetration of the cement into the bone at the periphery of the implant will result in a lower level of fixation of the implant and therefore a lower torsional stability of the implant. Finally, if there is a low degree of adhesion of the bone cement at the implant/bone interface then the peripheral region will also be open to ingress by foreign particles.
Commonly, a re-surfacing shell has a stem extending along the polar axis, which is received in a bore in the head of the bone along the polar axis of the head. It has been found that cementing techniques employing low viscosity cements have the further disadvantage that, when the re-surfacing implant is fitted, the bone cement is drawn down with the stem of the implant during insertion. This displacement of the bone cement potentially leads to a more secure fixation of the stem of the implant than the implant itself. This has an additional clinical disadvantage of stress shielding which could result in a subsequent loss of bone adjacent to the implant underside.
It has therefore been found to be desirable to use high viscosity cements. However, cementing techniques employing low viscosity cements cannot be used with high viscosity bone cements as the forces required to extrude the excess bone cement of high viscosity from the head of a bone have been found to be too large. This has made it difficult for such techniques to provide a uniform distribution of high viscosity bone cement on the surface of the head of the bone, so that the correct seating of the implant can be difficult to achieve.
Accordingly, alternative techniques were required to be developed for use with high viscosity cements. One such technique is disclosed in WO-2006/054062. While the technique disclosed in WO-2006/054062 provides an effective means for controlling the distribution of bone cement on the surface of the head of a bone to prepare the head of the bone to receive a re-surfacing prosthesis, it has been found to be desirable to develop further alternative techniques suitable for use with high viscosity cements.